Electron discharge device



Feb. 13, 1940. H. BRUINING ELECTRON DISCHARGE DEVICE Filed March 17, 1958 IN VEN TOR.

HAJO BRUINING EBY TTORNEYS.

45 the primary emitter.

Patented Feb. 13, 1940 UNITED STATES 2,189,973 ELECTRON DISCHARGE DEVICE Hajo Bruining, Eindhoven, Netherlands, assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application March 17, 1938, Serial No. 196,341 In the Netherlands April 13, 1937 1 Claim.

less than unity and therefore emitscomparatively 8 few secondary electrons when struck by a current of primary electrons.

The use of electron discharge tubes is sometimes attended with difficulties because electrons emitted by the cathode impinge at high speed on 10 other parts of the tube, such as the cold electrodes, and dislodge secondary electrons from. the surface of these parts. To minimize this secondary emission, the surface from which these secf ondary electrons emerge may be coated with a substance having the property of emitting only a very small quantity of electrons when struck by a current of electrons. For example, the grids or anodes of electron discharge tubes have been coated with oxides of various metals, such as chromium, silver, nickel, molybdenum, and so forth, and also extensive use is made of a coating of carbon which, especially in the form of soot, has very low secondary emission.

The principal object of the invention is to provide an electron discharge tube in which sec ondary electron emission from tube parts is very low, and more particularly to provide a cold electrode which has very low secondary electron emission.

An electron discharge tube constructed according to the present invention comprises one or more cold electrodes which are at high positive potential during operation of the tube, and which have the surface subject to electron bombard- 5 ment covered with pure metallic lithium. Lithium appears to have a very low coefiicient of secondary emission, possibly because electrons which strike a surface covered with a metal of atomic weight of less than 10 penetrate to a comparatively great depth into this surface so that the secondary electrons cannot easily emerge.

It is well known that the secondary emission of every material varies with the potential set up between the electrode bearing this material and When the secondary emission is graphically plotted as a function of this potential, the resulting curve has a maximum indicating that with all material the secondary emission increases with'the voltage until a definite voltage is attained and subsequently decreases again. The maximum secondary electron emission of lithium and beryllium occurs at comparatively low voltages, and the maximum itself is low. The coefficient of secondary electron 5 emission, which is the number of secondary electrons emitted per primary impinging electron,is always less than unity. The maximum emission of lithium is 0.5 secondary per primary and occurs at volts. This metal is therefore eminently suitable for use as coatings on electrodes 6 which are at high potential during operation of the tube, and when so used it has a secondary emission even lower than the materials most generally used for this purpose, such as soot, aquadag, and the like. This metal is particularly advan- 10 tageous for use in cathode ray tubes and the like, where the potentials generally exceed 200 volts.

For a better understanding of the invention, reference may be had to the accompanying drawing, which shows more or less diagrammatically 15 in longitudinal section a conventional four-electrode screen grid tube embodying the invention.

In the specific embodiment of the invention shown in the drawing the screen grid tube comprises the conventional bulb l which is highly go evacuated and which encloses an electrode assembly of conventional construction comprising an indirectly heated oxide, coated thermionic cathode 2, a control grid 3, a screen grid 4, which in operation is a grid-like anode, and a tubular 25 main anode 5 surrounding the screen gridand the cathode. The various electrodes are, supported by narrow end spacers, usually of mica. As such tubes are usually operated with an anode potential of 200 volts or higher, secondary elec- 3o tron'emission from the anode is a serious problem. In accordance with the invention, the inner surface of the main anode 5, usually made of nickel, is covered or coated with a layer 6 of a pure lithium. A convenient way of obtaining 5,

this layer is to vaporize a pellet 1 of "lithium contained in a nickel cylinder 8 which is heated by. a tungsten wire heating coil 9. The vaporized lithium deposits principally on the inner sur face of the nickel anode 5. It is usually advang tageous to maintainthe cathode hot during the vaporization of the lithium to hinder deposition of the lithium on the cathode. The control grid may also be kept hot if desired, although in operation the control grid usually attains atem- 5 perature at which any lithium on it is driven oif, as lithium melts at approximately C.

As the lithium should be deposited on the anode 5 in a very pure form, the tubefshould be very well exhausted when the lithium is vaporized. 50 The tube may be exhausted in the conventional manner, and the vacuum perfected by vaporizing the usual getter carried on the conventional getter tab l0, before the lithium is vaporized. A pellet? of clean lithium may be secured by 55,

cutting a piece of lithium from a lump of the metal, and quickly putting it into the nickel cylinder 8 where it is partially protected from the air. Care should be taken that the nickel cylinder 8 and the lithium in it are kept as cool as possible during exhaust consistent with thorough degassing of the metals in the tube.

The lithium may be applied to the cold electrodes of the tube by vaporization, by cathode disintegration, or by other known methods of introducing metal vapor into an evacuated container.

The coating of lithium should be thick enough so that the coefiicient of secondary electron emis' sion of the coated anode is that of lithium. In coating an anode with lithium it will be found that the secondary emission at 200 volts is at first the secondary emission of the metal of the anode, then rises as a very thin lithium coating is deposited, and then falls to that of the deposited metal as the coating attains a greater thickness. By observing the decrease in secondary emission as the thickness of the layer of deposited metal increases the necessary thickness of the coating of lithium can be determined.

While I have illustrated my invention as embodied in a tetrode or screen grid tube, it is not limited to such tubes, but is useful in any tube having an anode electrode maintained at a potential of 200 volts or more, and particularly in cathode ray tubes and similar devices where anode electrodes are operated at high positive potentials.

I claim:

A thermionic electron discharge device for operating solely by thermionic emission comprising a thermionic cathode constituting the sole source of electron emission during normal operation, and a cold anode electrode having the surface facing said thermionic cathode covered by a layer of pure lithium of sufficient thickness to make the coefiicient of secondary electron emission from said surface the same as that of pure lithium.

HAJO BRUINING. 

